linux-kernel - Re: Terrible performance of sequential O_DIRECT 4k writes in SAN environment. ~3 times slower then Solars 10 with the same HBA/Storage.

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Date:	Wed, 8 Jan 2014 06:03:43 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Sergey Meirovich <rathamahata@...il.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Gluk <git.user@...il.com>
Subject: Re: Terrible performance of sequential O_DIRECT 4k writes in SAN
 environment. ~3 times slower then Solars 10 with the same HBA/Storage.

On Wed, Jan 08, 2014 at 02:17:13AM +0100, Jan Kara wrote:
>   Well, I was specifically worried about i_mutex locking. In particular:
> Before we report appending IO completion we need to update i_size.
> To update i_size we need to grab i_mutex.
> 
> Now this is unpleasant because inode_dio_wait() happens under i_mutex so
> the above would create lock inversion. And we cannot really do
> inode_dio_done() before grabbing i_mutex as that would open interesting
> races between truncate decreasing i_size and DIO increasing it.

Yeah, XFS splits this between the ilock and iolock, which just makes
life in this area a whole lot easier.

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